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There are over 1,000 different species of killifish that belong to several different families. The vast majority of killifish are small, up to about 5 cm (2 in) long. Since there are so many different types of killfish, the group has a wide variety, with species found in Africa, southern Europe, and the Americas. All species are oviparous, which means that the mother releases undeveloped eggs. Most species have interesting colors and are also easy to keep in aquariums, making them popular as pets.
The largest family is Rivulidae, which contains more than 300 species. There are several other families such as Valenciidae, Aplocheilidae, and Cyprinodontidae. In some areas, the term killfish is used to describe any fish that is part of the Cyprinodontidae family, while in other regions it has a much broader meaning.
In general, killifish are small and grow a few centimeters. However, there are some species that do grow. The largest species grows to about 15 cm in size. This is another reason why fish are regularly kept in aquariums; they require less space compared to many other types of sea creatures. Usually the fish lives about two years.
The variety of this type of fish is great, since different species are found in the waters of most continents. Antarctica and Australia are the only two continents where no species can be found, but northern Europe also has no killfish. Most species live in lakes and rivers, but there are others that have adapted to more temporary areas of water, such as floodplains.
All killifish are oviparous, which means that they lay eggs. To be classified as an oviparous fish, the mother must release eggs before significant development occurs within her. Most, but not all, fish are kept young this way.
Killfish are known for being easy to keep as pets. Fish can adapt, and even reproduce, in an artificial environment with little trouble. A potential problem with some species is that they can become aggressive towards other species, so it is important to mix fish carefully. Most species are brightly colored, making them especially attractive to fish lovers. One particularly common species is the Golden Wonderor, also known as the Striped Panchax, which can often be found in pet stores as well as specialty aquarium fish dealers.